Spiral slot antenna



Dec. 2; 1958 v TURNER 2,863,145

. SPIRAL SLO'I KANTENNA Filed Oct. 19. 1955 IN V EN TOR.

[aw/1v m. 72/ 4/56 WW v w Ma i United States Patent SPIRALSLOT ANTENNA Edwin M. Turner, Dayton, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Application October 19, 1955, Serial No. 541,551

3 Claims. (Cl. 343-767) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to an antenna designed to cover in a minimum number of antennas the V. H. F. and U. H. F. frequency bands from 30 to 15,000 mc./ second, and to be suitable for flush mounting on high speed aircraft, for built in installation in television receivers, and for other communications and navigation applications where bandwidth, pattern stability, size and weight are critical.

Briefly, the antenna consists of an open transmission line coiled in a spiral configuration and mounted on a dielectric support which may, for example, be made of glass or plastic. The antenna is preferably fed from the center of the spiral and is preferably backed by a shallow reflecting cavity to give unidirectional prpagation characteristics. The geometrical properties of the spiral are such as to give such an antenna electrical properties resulting in a bandwidth characteristic substantially wider than previously known types of high gain antennas. It has been found that the radiation pattern and impedance of such an antenna can be made practically independent of frequency over a band covering a wide frequency ratio. The bandwidth may be as great as 10:1 depending upon the standards used. Furthermore the antenna has very good efliciency, is circularly polarized, and has a very desirable radiation pattern.

It is an object of this invention to provide a U. H. F.- V. H. F. antenna which has very wide bandwidth characteristics and receives signals of any polarization.

It is a further object of the invention to provide such an antenna which is suitable for flush mounting, is small in size and weight, and can be made impervious to humidity, temperature, altitude, sand or dust.

It is a still further object of the invention to provide such an antenna which is adaptable to be manufactured by printed circuit techniques and can be identically mass reproduced at small cost.

These and other objects and advantages as well as the manner of construction and operation of the antenna will be more fully understood from the following specification and drawings wherein;

Figure 1 is a front isometric view of a cavity backed spiral slot antenna.

Figure 2 is a rear isometric view of the antenna of Figure 1.

Figure 3 is a front isometric view of a disc mounted spiral slot antenna.

Turning now to Figures 1 and 2 there isshown for purposes of illustration a transmission line consisting of conductors 1 and 2 which are fed at terminals 3 and 4 respectively and are coiled in the configuration of concentric Archimedian spirals. Conductors 1 and 2 are mounted by any suitable means on a dielectric supporting surface 5 which may for example be a plastic disc as Shown or could consist of ceramic posts. Conductors 1 2,863,145 Patented Dec. 2, 1958 and 2 may be stamped from sheet metal and screwed to disc 5 or they may be painted, sprayed, or printed on or imbedded in dielectric sheet 5, or mounted on standofi insulators. As shown in the drawings, the antenna is backed by a shallow cylindrical ,cavity having a side wall 6 and rear wall 7, both being constructed in any suitable manner of metallic or other low loss material which will reflect electromagnetic radiation. A standard coaxial line fitting 8 is mounted on rear wall 7 and is connected through the cavity in any suitable manner to the feed points 3 and 4.

As shown in Figure 3, a similar antenna consisting of conductors 11 and 12 mounted on a dielectric surface or slab 15 and fed at points 13 and 14 may also be used without being backed by a reflecting gravity. In this case disc 15 would be mounted in any manner suitable for the intended application of the antenna and points 13 and 14 would be fed in any conventional or desirable manner. The spiral antenna can be fed from the outer end of the conductors but experience has indicated that the center feed is preferable. Of course, the unidirectional character of the antenna is lost if the reflecting cavity is not used.

It will be noted that conductors 11 and 12 are shown as being wider than conductors 1 and 2 with the result that the gap, space, or slot between conductors 11 and 12 is narrower than that between conductors 1 and 2. The size and material of the conductor, the number of turns and type of spiral, the dielectric constant of the mounting disc, the type of feed selected, and the size and shape of the reflecting cavity or means are all design factors which in any particular instance or application will affect the power handling capacity of the antenna as well as its efficiency, pattern, impedance and other characteristics.

Furthermore the type of spiral and the type of cavity used are not critical to good radiation but are also design parameters. Satisfactory results have been obtained, for example, with a rectangular spiral. The cavity may be either resonant or nonresonant or various other types of reflectors could be used such as a parabolic reflector with the spiral mounted at its focal point.

A complete analytical theory of the operation of the spiral slot type of antenna has not yet been established conclusively. One plausible approach to an explanation of the action of a many tu'rn spiral would be as follows: As a traveling wave of current emanates from the center and propagates outwardly along the spiral it will reach some region where the distance along one complete turn of the spiral conductor is equal to one wavelength at the frequency of excitation. At such a region currents in adjacent conductors will be in phase. If there are many turns in the spiral and if the pitch of the: spiral is small, there will in fact be several turns for which the currents in adjacent conductors will be nearly in phase. Mutual reinforcement of the electromagnetic field in the current band consisting of the region of these in-phase currents results in a marked increase in attenuation of the traveling wave due to radiation. Since the current elements at any given point on the spiral which are in space quadra ture are also in phase quadrature in such a current band, the radiation from the system will be approximately circularly polarized with the same rotational sense as that of the spiral.

Of course, the number of turns shown is illustrative only. Good radiation can be achieved with either a small or large number of turns, but the patterns will vary with the number of turns. A full explanation for the action of a spiral antenna of a small number of turns is not readily apparent.

It is obvious that as the frequency of the exciting wave applied to a many turn spiral is changed the effect is to change the position of the current band along a radial line from the center of the spiral. Due to this position adjustment the characteristics of the antenna are virtually independent of frequency. Since the circumference of a circle (corresponding closely to one turn of the spiral or to one'wavelength) is equal to'pi. times the diameter of a circle, it is apparent that the diameter of the spiral at the radiating current band-will be equal to the wavelength divided by pi. This relationship of course determines the minimum size of a useful many turn spiral antenna for any desired frequency range.

The lower frequency limit for circularly polarized radiation is set by the maximum diameter of the spiral, which must be at least equal to wavelength divided by pi. The upper frequency limit of a wide band closely spaced spiral has not been accurately determined and there is nothing in the basic theory that would set an upper limit. As the frequency is increased, however, the position of the radiating current band will move in close around the feed points, and the configuration of the feeding arrangement will become extremely important.

Both from a theoretical investigation and from cxperimental measurements it can be shown that the spiral slot antenna is essentially a constant beam width antenna independent of frequency. The Wide band properties of the spiral antenna do not appear to depend upon the phase velocity variation that is commonly associated with the modes of helical antennas, but rather seem to derive from the fact that the effective physical aperture of the spiral antenna varies inversely with frequency as explained in terms of the current band theory.

If a cavity backing is used the radiation will not only be essentially axial but also unidirectional. It is desirable that the cavity be shallow, preferably not greater than one quarter wavelength at the highest frequency to be used, so that an essentially uniform current distribution exists inside the cavity. For most stable performance resonance should be avoided.

Thus it is seen that Ihave provided an antenna suitable for use in the V. H. F.-U. H. F. bands which may be flush mounted for zero drag on high speed aircraft and which is admirably suited to any application where size and weight are critical factors. Furthermore the radiation pattern and impedance of the antenna are substantion, it will be understood that various modifications may he made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An antenna comprising a plural conductor open transmission line, the conductors of said open transmission line being coiled in concentric Archimedien spirals, an essentially flat dielectric mounting surface, said spirals being mounted on said surface, means to feed the conductors of said transmission line at the center of said spirals, said dielectric surface forming the front wall of a reflecting cavity, the other walls of said cavity being constructed of a material which reflects electromagnetic radiation.

2. Apparatus as in claim 1 wherein the depth of said cavity in a direction along the axis of said spiral is small by comparison to a wavelength at the highest frequency to be fed to said antenna, whereby said cavity is nonresonant.

3. A wide band width directional antenna comprising a substantially flat dielectric mounting surface, a pair of conductors mounted on said mounting surface, said conductors being arranged in coplanar concentric Archimedien spirals the plane of said spirals being substantially parallel to said surface, feed conductors connected to the inner ends of said pair of conductors and a reflector mounted in axially spaced relation to said spirally arranged conductors.

References Cited in the file of this patent UNITED STATES PATENTS 

